fs/direct-io.c: minor cleanups in do_blockdev_direct_IO

We already get the block counts and calculate the end block at the
beginning of the function.  Let's use the local variables for
consistency and readability.  No functional changes

[akpm@linux-foundation.org: constify the locals to prevent future slipups]
Link: http://lkml.kernel.org/r/1519638870-17756-1-git-send-email-nborisov@suse.com
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>